1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, which in particular may have a high withstand voltage element and a low withstand voltage element mounted together on the same semiconductor substrate.
2. Background Information
In recent years, liquid crystal displays have prevailed in the fields of personal computers and televisions, and their rapid growth in these fields is remarkable. Moreover, as liquid crystal displays have come to be used in cellular phones, digital cameras etc, it is expected that there will be even more demands for them in the future.
A conventional liquid crystal panel requires high voltage in order to be operated. Therefore, a driver LSI for driving the liquid crystal panel needs high withstand voltage MOS (metal-oxide semiconductor) transistors. On the other hand, a logic circuit for digital processing needs an advanced logic process in order to obtain processing speed.
Generally, in the logic processor, low withstand voltage MOS transistors are used. As opposed to this, in the driver LSI, elements having both high withstand voltage MOS transistors and low withstand voltage MOS transistors mounted on the same semiconductor substrate are used.
Examples of general types of semiconductor devices having high withstand voltage MOS transistors and low withstand voltage MOS transistors mounted together on the same semiconductor substrate are exhibited in Japanese Laid-Open Patent Application No. 2000-150665 (hereinafter to be referred to as Patent Reference 1) and Japanese Laid-Open Patent Application No. 2000-200836 (hereinafter to be referred to as Patent Reference 2). FIG. 1A and FIG. 1B are diagrams showing structures of a general type of conventional semiconductor device 900.
FIG. 1A is a sectional view of the conventional semiconductor device 900 taken along a line I-I′, and FIG. 1B is an overhead diagram of the semiconductor device 900. The I-I′ section of FIG. 1A is a section of the line I-I′ shown in FIG. 1B. Here, the same reference numbers are used for the same structural elements.
As shown in FIG. 1A and FIG. 1B, the semiconductor device 900 has a high withstand voltage MOS transistor region 900A and a low withstand voltage MOS transistor region 900B. A MOS transistor formed in the high withstand voltage MOS transistor region 900A (hereinafter to be referred to as high withstand voltage MOS transistor) has a gate oxide film 913a and a gate electrode 914 formed on a silicon substrate 911, sidewall spacers 916 formed on two sides of the gate electrode 914, and a pair of source/drain regions 915 sandwiching a region underneath the gate electrode 914 in the silicon substrate 911. On the other hand, like the high withstand voltage MOS transistor, a MOS transistor formed in the low withstand voltage MOS transistor region 900B (hereinafter to be referred to as low withstand voltage MOS transistor) has a gate oxide film 913b and a gate electrode 914 formed on the silicon substrate 911, sidewall spacers 916 formed on two sides of the gate electrode 914, and a pair of source/drain regions 915 sandwiching a region underneath the gate electrode 914 in the silicon substrate 911.
The high withstand voltage MOS transistor and the low withstand voltage MOS transistor are electrically separated from each other by a field oxide (a field oxide is also called an element isolating insulation film) 912 formed in the silicon substrate 911.
In the above structure, a boundary 900a between the high withstand voltage MOS transistor region 900A and the low withstand voltage MOS transistor region 900B is positioned on the field oxide 912.
Now, with reference to FIG. 2A to FIG. 3B, a method of manufacturing the semiconductor device 900 according to prior art will be explained. FIG. 2A to FIG. 3B show manufacturing processes focusing attention on the section of the line I-I′ shown in FIG. 1B.
First, as shown in FIG. 2A, field oxides 912 are formed in the p-type silicon substrate 911 using a well known STI (Shallow Trench Isolation) method for instance. By this arrangement, active regions and field regions are defined in the surface of the silicon substrate 911.
Next, by conducting a thermal oxidation treatment on the surface of the silicon substrate 911, a gate oxide film 913 for the high withstand voltage MOS transistor is formed on the entire surface of the silicon substrate 911 as shown in FIG. 2B. Here, the gate oxide film 913 is normally formed to a thickness which is sufficient to not be damaged by an operating voltage. Generally, the gate oxide film 913 is formed to the thickness of around 30 to 50 nm (nanometer) for instance.
Next, by conducting a known photolithographic process, a resist pattern R901 is formed only in the high withstand voltage MOS transistor region 900A. Then, the gate oxide film 913 in the low withstand voltage MOS transistor region 900B is removed by a known etching method while using the resist pattern R901 as a mask. By this process, the gate oxide film 913A which is a part of the gate oxide film 913 remains only in the high withstand MOS transistor region 900A as shown in FIG. 2C. The resist pattern R901 remained on the gate oxide film 913A is removed after the etching process is over.
Next, by conducting a thermal oxidation treatment on the entire surface of the silicon substrate 911, a gate oxide film 913B for the low withstand voltage MOS transistor is formed in the low withstand voltage MOS transistor region 900B as shown in FIG. 2D. Here, the gate oxide film 913B is normally formed to a thickness which is decided depending on the operating voltage and performance expected from the low withstand voltage MOS transistor. Generally, the gate oxide film 913B is formed to a thickness of around 2 to 7 nm for instance.
Next, polysilicon is deposited on the entire surface of the silicon substrate 911 on which the gate oxide films 913A and 913B are formed, and processed by a known photolithographic method and an etching method to form the gate electrode 914 on the gate oxide film 913A in the high withstand voltage MOS transistor region 900A and the gate electrode 914 on the gate oxide film 913B in the low withstand voltage MOS transistor region 900B. Then, while using the gate electrodes 914 as masks, an etch back process is done on the entire surface of the silicon substrate 911 to remove the gate oxide films 913A and 913B except for the parts underneath the gate electrodes 914. By these processes, a structure shown in FIG. 3A can be obtained.
Next, an insulation film such as a silicon oxide film or a silicon nitride film is formed on the entire surface of the silicon substrate 911 using a known CVD (Chemical Vapor Deposition) method, after which an etch back process according to a known etching technique is performed on the insulation film to form the sidewall spacers 916 on the sides of the gate electrodes 914 respectively, as shown in FIG. 3B.
Next, arsenic (As) ions are implanted into the silicon substrate 911 while using the field oxides 912, gate electrodes 914 and the sidewall spacers 916 as masks, a pair of source/drain regions 915 are formed in the active region of each of the high withstand voltage MOS transistor region 900A and the low withstand voltage MOS transistor region 900B in a self-aligning manner, the pair of source/drain regions 915 being formed in a way sandwiching a region underneath the gate electrode 914 and the sidewall spacers 916.
Taking the processes described above, a semiconductor device having a low withstand voltage transistor and a high withstand voltage transistor formed on the same semiconductor substrate can be produced.
However, in the above-described conventional manufacturing method, it has been noted as a problem that a step is produced in the upper part of the field oxide in the boundary between the high withstand voltage MOS transistor region and the low withstand voltage MOS transistor region. This is because in the etching process of FIG. 2C, over etching to the extent of about several dozen percent of the thickness of the gate oxide film 913 is done for the purpose of preventing variations in thickness to be made in the gate oxide film 913B after etching process. Due to such over etching, the upper part of the field oxide 912 which is not covered by the resist pattern R901 is also etched as shown in FIG. 2C. As a result, a step is formed in the upper part of the field oxide 912 in the boundary between the high withstand voltage MOS transistor region 900A and the low withstand voltage MOS transistor region 900B as can be seen in FIG. 2C. Normally, this step is about 50 to 100 nm high, although it depends on the thickness of the gate oxide film 913.
Such a step can be a cause of defective printing in the photolithographic process in forming the gate electrode 914 in the later process, and can be a cause of etching residuals of the polysilicon film. In addition to that, since the field oxide 912 becomes thinner, leakages, for instance, between transistors and wirings may be caused (hereinafter to be referred to as inter-field leakage).
In the above described way, when there is a step in the upper part of the field oxide, problems such as open, short, leakage, etc. can occur, which leads to a problem in which normal operation of the semiconductor device becomes difficult.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved method of manufacturing a semiconductor device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.